1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a code combining soft handoff (CCSH) method.
2. Background of the Related Art
In mobile communications, a handoff is an automatic switchover of the current traffic channel that occurs when a mobile terminal moves from one cell to another cell. Such a handoff is typically classified into a hard handoff and a soft handoff. The hard handoff first breaks the existing communication channel before opening a new traffic channel, and the soft handoff first connects a new channel and then breaks the existing channel.
In other words, according to the hard handoff, when a mobile communication terminal (i.e., mobile station) moves from one base station to another base station, the channel connected to the existing base station is broken, and then a new channel of the new base station is connected.
According to the soft handoff that is a handoff between cells, when a mobile communication terminal (i.e., mobile station) comes near another cell region having the same frequency while the mobile terminal is in use, the mobile terminal detects and informs the current cell that the signal strength of the pilot signal of a new cell is sufficiently high, and the new cell opens a traffic channel for the mobile terminal with the new base station. At this time, the mobile terminal simultaneously maintains communication through the traffic channels of the two cells.
As the mobile terminal approaches the new cell, the signal of the previous cell becomes weaker. If the strength of this previous signal is lower than a predetermined level, the mobile terminal informs the two cells of this fact, and then the previous cell breaks the communication channel through the mobile station. The mobile terminal thus continues the call through the new cell of a good signal state.
A softer handoff is a handoff between different sectors of one cell. According to the softer handoff, one cell is divided into several sectors, and when the mobile terminal in use moves from one sector to another sector within the same base station, the traffic channel is connected in the same manner as the soft handoff.
Meanwhile, a high-speed data transmitting system uses a CCSH (also known as a code combining and packet combining) handoff method using turbo coding. According to the CCSH, one signal is encoded by different coding methods, and resultant coded signals are transmitted. A receiving part decodes and combines the signals encoded by the different coding methods to obtain a gain. According to the CCSH handoff method, several base stations transmit the signals encoded by different coding methods from the various base stations and a mobile terminal receives and combines the signals encoded by the different coding methods to obtain the gain. If the mobile terminal receives the signals encoded by the same coding method, however, it obtains no gain.
Specifically, as shown in FIG. 1, data to be transmitted to a base station is outputted from a master switch center 101 of a base station controller. The data is then inputted to a turbo encoder 101b, which encodes the data. The turbo encoder 101b then outputs two signals encoded with different patterns.
The two signals are transmitted to the respective base stations with pilot signals from respective pilot channel sections 102a and 102b included therein. The transmitted signals are then received by the mobile terminal from each of the respective base stations. The mobile terminal decodes and combines the two signals encoded with different patterns to obtain the gain.
Meanwhile, according to the CCSH handoff method, the respective base station is allocated with a PN number for discrimination among base stations and cell regions that transmit/receive signals to/from the mobile terminal. In transmitting/receiving the signals to/from the mobile terminal, the respective base stations are under the control of the base station controller, and have different code patterns.
Here, the mobile terminal receives the signals with specified code patterns from all the base stations located in the cell region where the mobile terminal itself is included. Since two code patterns can be used in the CCSH handoff method, the more than 3-way handoff method allocates two code patterns with the permission of overlapping of the respective base stations.
Specifically, when the mobile terminal moves between cells, it moves through a 2-way or 3-way handoff regions where two or three cells of the base stations overlap. Since two code patterns are allocated to the base station, one of the two code patterns is allocated if the signal is received from one base station. If the signal is received from two base stations, the two code patterns are allocated to the two base stations, respectively. Thus, the mobile terminal receives a first code pattern from the first base station and a second code pattern from the second base station.
If the signal is received from three base stations, however, one of the two code patterns is allocated twice. That is, two code patterns are allocated with the permission of overlapping of the respective base stations.
The 2-way or 3-way region indicates the number of base stations from which the mobile terminal simultaneously receives the signal. Thus, a mobile terminal receives a signal from two base stations in a 2-way region, and receives a signal from three base stations in a 3-way region. The number of base stations from which a mobile terminal receives a signal is determined in accordance with the level of the pilot signal received from the respective base station. If the level of the pilot signal received from the base station is higher than a predetermined value, the mobile terminal additionally receives the signal of the corresponding base station, while if the level of the pilot signal is below the predetermined level, the mobile terminal drops the signal of the base station not to be received. The 2-way or 3-way region is thus determined.
Now, a related code combining handoff method will be explained with reference to the accompanying drawings.
Referring to FIGS. 2, 3, and 4, A to C base stations 201, 202, and 203 are allocated with PN numbers a, b, and c, respectively, for discrimination among base stations. In neighboring portions of the base stations exist 2-way (210a, 210b, and 210c) and 3-way (220) handoff regions where the signal is received from two and three of the base stations 201, 202, and 203, respectively.
First, if the mobile terminal 205 in the cell region of the A base station 201 is in an operation state, it receives the signal from the A base station 201, is allocated with a code pattern α, and thus receives the signal of the A base station 201 with the code pattern α.
At this time, the mobile terminal 205 receives an extended supplemental channel assignment message (ESCAM) from a new base station, and determines with which code pattern the mobile terminal 205 and the base station initially communicate. The ESCAM includes a PILOT_PN field and a PUNCTURE_PATTERN (also called CCSH_TYPE) field. The PILOT_PN field includes the PN code allocated to the respective base stations 201, 202, and 203, and discriminates from which base station the ESCAM message is received. The PUNCTURE_PATTERN field informs which code pattern the corresponding base station uses. For example, for discrimination among the base stations, the A base station 201 is allocated with the PN number a, the B base station 202 with the PN number b, and the C base station 203 with the PN number c. Also, if information defined in the PUNCTURE_PATTERN is 00, no code pattern is used. If the information is 01, the code pattern α is allocated as the code pattern of the base station that transmitted the ESCAM, while if the information is 10, a code pattern β is allocated as the code pattern of the base station that transmitted the ESCAM. It should be understood that the code pattern could be a puncture code pattern, or any other coding scheme.
Since there are two code patterns, at least three cases including an unusable state should be defined. Accordingly, the PUNCTURE_PATTERN field requires at least 2 bits.
Though the signal is received from the B base station 202 and the C base station 203, the mobile terminal disregards this since the signal is weak.
Meanwhile, as the mobile terminal 205 gradually approaches the B base station 202, the pilot signal strength of the signal received from the B base station 202 becomes greater. The mobile terminal 205 detects whether the pilot signal strength of the signal received from the B base station 202 is higher than a specified value, and if the pilot signal strength of the B base station 202 becomes higher than the specified value, the mobile terminal 205 transmits an extended pilot strength measurement message (EPSMM) to the respective base stations to indicate as such, and allocates the code pattern β to the B base station 202. The respective base stations transmit an universal handoff direction message (UHDM) to the mobile terminal 205 to inform this. The mobile terminal 205 receives the code pattern α (or of a default encoder type) from the A base station 201, receives the code pattern β (or of a complementary encoder type) from the B base station 202, and transmits an extended handoff completion message (EHCM) to the respective base stations to complete the handoff.
Here, the fact that the pilot signal of the B base station 202 becomes higher than the specified value means that the mobile terminal 205 at least enters into the 2-way handoff region 210a where the A base station 201 and the B base station 202 overlap.
If the mobile terminal 205, which is in the 2-way handoff region 210a of the A base station 201 and the B base station 202, gradually approaches the C base station 203, the strength of the signal received from the C base station 203 becomes greater. If the pilot signal strength of the C base station 203 becomes higher than the specified value, the mobile terminal 205 transmits an EPSMM to the respective base stations to inform this, and allocates one of the code patterns, which were allocated to the A base station 201 or to the B base station 202, to the C base station 203. The respective base stations transmit an UHDM to the mobile terminal 205 to inform this. The mobile terminal 205 receives the signal from the C base station 203, and transmits an EHCM to the respective base stations to complete the handoff.
Here, the fact that the pilot signal of the C base station 203 becomes higher than the specified value means that the mobile terminal 205 enters into the 3-way handoff region 220 where the A base station 201, B base station 202, and C base station 203 overlap.
If the code pattern α is allocated to the C base station 203, the mobile terminal 205 receives the signal of the code pattern α from the A base station 201 and the C base station 203, and receives the signal of the code pattern β from the B base station 202.
At this time, the mobile terminal 205 receives, decodes, and combines the signals of different code patterns α and β, and thus a signal of a better quality can be produced by the diversity effect to obtain a gain.
The allocation of the code patterns to the base stations is performed by the base station controller (not illustrated) which receives information from the respective base stations. That is, the base station controller allocates different code patterns in the 2-way handoff region, and allocates two code patterns to three base stations with the permission of overlapping in the 3-way handoff region.
Here, if the mobile terminal 205 in the 3-way handoff region 220 moves to the cell region of the A, B, or C base station 201, 202, or 203, only one code pattern corresponding to the base station remains, and the signals of the remaining two base stations are dropped because the level of the pilot signals of the two base stations becomes lower than the specified value. The mobile terminal consequently receives the signal from the corresponding remaining base station only.
If the mobile terminal 205 in the 3-way handoff region 220 moves to the 2-way handoff region 210a of the A and B base stations 201 and 202, or to the 2-way handoff region 210b of the B and C base stations 202 and 203, the respective signal of the C base station 203 or of the A base station 201 is dropped, and thus the mobile terminal 205 receives the signals from the B and C base stations 202 and 203, or from the A and B base stations 201 and 202.
At this time, since the A and C base stations 201 and 203 use the code pattern α, and the B base station uses the code pattern β, there exists no problem in performing the code combining handoff method that receives, decodes, and combines the signals of different code patterns, and obtains a gain.
If, however, the mobile terminal 205 in the 3-way handoff region 220 moves to the 2-way handoff region 210c of the A and C base stations 201 and 203, the signal from the B base station 202 is dropped, and the mobile terminal 205 receives the signals from the A and C base stations 201 and 203. In this case, since both the A and C base stations 201 and 203 use the code pattern α, no gain can be obtained by the code combining handoff method.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.